1. Field of the Invention
The present invention relates to an optical interconnection receiving module. The present invention particularly relates to a technology for preventing the effect of cross talk noise caused by an amplifier of another channel in a photoelectric conversion circuit dealing with a plurality of channel signals and an optical interconnection receiving module including the photoelectric conversion circuit. The optical interconnection receiving module according to the present invention is used in an optical communication system, and is useful in converting an optical signal into an electric signal and in amplifying and distinguishing the electric signal.
2. Related Arts
Conventionally, as shown in FIG. 1, at an apparatus on the transmitting side in an optical communication system, information to be transmitted is converted from an electric signal to an optical signal by a transmission module 100 having a laser diode array 120 and the like, and the information is transmitted to an apparatus on the receiving side via an optical fiber array 200. At the apparatus on the receiving side, the optical signal received by an optical interconnection receiving module 300 having a photodiode array 320 and the like is converted to an electric signal and sent to an interface circuit of a microcomputer system and the like.
FIG. 9 shows a basic configuration of an input distinguishing circuit IDC provided on the input terminal side of a receiving IC (semiconductor integrated circuit) in the optical interconnection receiving module 300. In the input distinguishing circuit shown in the figure, an electric signal converted from an optical signal by a photodiode 321 is amplified at a preamplifier 331 and supplied to a comparator 332. At the comparator 332, the signal is compared with reference voltage Vref to be distinguished as or xe2x80x9c1xe2x80x9d. Also, an amplifier 333 having the same type of circuit as that of the preamplifier 331 is used as a circuit for generating the reference voltage Vref. This makes it possible to provide high resistance to power supply noise.
It is possible to provide high resistance to power, supply noise for the following reason. If the amplifier 333 for generating the reference voltage Vref has the same type of circuit as that of the preamplifier 331, the noises accompanying the two inputs from the two amplifiers to the comparator 332 are in phase with each other. Therefore, the effect of supply voltage noise will not appear in the output of the.comparator 332. Specifically, in the case where the reference amplifier 333 has the same type of circuit as that of the preamplifier 331, even if noise appears in the output of the preamplifier 331 due to the effect of the noise carried with the supply voltage Vcc, the same noise also appears in the output of the reference amplifier 333, which is supplied with the common supply voltage Vcc. Therefore, the relative correlation between the two inputs of the comparator 332 is not changed, that is, the noises accompanying the two inputs from the two amplifiers to the comparator 332 are in phase with each other. Thus, no effect of, supply voltage noise will appear in the output of the comparator 332.
However, in the circuit shown in FIG. 9, the input terminal of the preamplifier 331 is connected with the photodiode 321, while the input terminal of the reference amplifier 333 is not connected with the photodiode 321. Therefore, the circuit shown in FIG. 9 is not perfectly symmetrical. Thus, it is equivalent to the fact that a capacitance is connected between the input terminal of the preamplifier 331 and the supply voltage terminal Vcc, and no capacitance is connected to the input terminal of the reference amplifier 333. Therefore, power supply noise comes into the input terminal of the preamplifier 331 via the photodiode 321, while such supply voltage noise does not come into the input terminal of the reference amplifier 333. Specifically, in terms of alternating current, the input frequency response characteristic of the preamplifier 331 and the input frequency response characteristic of the reference amplifier 333 with respect to power supply noise differ from each other, as shown in FIG. 10A. In the figure, a dotted line denotes the frequency characteristics of the output of the preamplifier, while a solid line denotes the frequency characteristics of the output of the reference amplifier. Thus, in the circuit shown in FIG. 9, the response to cross talk noise caused by the input distinguishing circuit of another channel differs between the preamplifier 331 and the reference amplifier 333. As a result, as the cross talk noise is increased, the noise that has come into the preamplifier 331 might cause the output of the preamplifier 331 to exceed the reference voltage Vref supplied from the reference amplifier 333, and the comparator 332 might wrongly distinguish the signal.
It is desirable especially in a module that receives a plurality of channel signals to have a receiving semiconductor integrated circuit that can deal with a wide range of input signals, including a small input signal that causes a current of a few xcexcA to flow through the photodiode as well as a large input signal that causes a current of a few mA to flow through the photodiode. However, on a channel where a large signal comes in, a current of a few mA flows through the bonding wire that connects the power supply terminal and the photodiode, and therefore a relatively large noise occurs in the supply voltage due to the inductance component of the bonding wire. The circuit needs to be designed in such a way that it can correctly distinguish signals even when the noise mentioned above comes through a power supply line into the preamplifier and the reference amplifier of a channel where only a current of a few xcexcA flows.
Incidentally, regarding a module for photoelectric conversion, there is proposed an invention according to which a capacitance equivalent to that of the photodiode is connected between the input terminal of a dummy amplifier and the supply voltage terminal, so that the effect of noise attributed to variations in power supply and temperature can be cancelled out by the preamplifier side and the dummy amplifier side. An example of this is found in, for example, Japanese Patent Laid-open No. Hei 8-139342. Incidentally, the dummy amplifier mentioned above corresponds to the reference amplifier according to the present invention. The invention mentioned above is similar to the present invention in that the effect of noise is cancelled out, but it does not take into consideration a module that receives multi-channel signals. Thus, the embodiments of the above invention were not sufficient in terms of the prevention of the effect of cross talk noise produced from a channel where a large current of a few mA flows to a channel where only a small current of a few xcexcA flows. Moreover, according to the prior invention mentioned above, a discrete capacitor is used as equivalent capacitance connected to the input terminal of the dummy amplifier, and therefore the above invention has the disadvantage of increasing the size of the module. The resulting size of a multi-channel module, in particular, may be fatally large for a product.
An object of the present invention is to provide an optical interconnection receiving module that can accurately distinguish an input signal by canceling out the effect of cross talk noise caused by the input signal of another channel on the preamplifier side and the reference amplifier side.
Another object of the present invention is to provide suitable device structures for a bypass capacitor used to stabilize supply voltage and for a capacitance device used to cancel out the effect of cross talk noise caused by the input signal of another channel on the preamplifier side and on the reference amplifier side, and thus reduce the size of the optical interconnection receiving module.
The above and other objects and new features of the present invention will be made clear from the descriptions of the present specification and the accompanying drawings.
Typical aspects of the present invention disclosed herein will be summarized as follows.
Specifically, according to a first typical aspect of the present invention, there are provided: a photodiode array comprising a plurality of photodiodes, each of which receives an optical signal on a plurality of channels and converts the optical signal into an electric signal; a preamplifier for amplifying the electric signal converted from the optical signal by the photodiode array for each of the channels; a comparator for distinguishing the amplified received signal; and a reference amplifier comprising the same type of circuit as that of the preamplifier which generates reference voltage supplied to the comparator; wherein at least the preamplifier, the comparator, and the reference amplifier are formed on a single semiconductor chip; and wherein a plurality of capacitance devices each equivalent to the photodiode are formed on the semiconductor chip, and each of the equivalent capacitance devices is connected between the input terminal of the reference amplifier and a supply voltage terminal.
According to the means mentioned above, even if cross talk noise caused by the input signal of another channel comes into the preamplifier via the photodiode, the noise of the same magnitude as that of the cross talk noise comes into the reference amplifier. Therefore, the noises of the inputs of the comparator are in phase with each other. Thus, the two noises on the preamplifier side inputted to the preamplifier and on the reference amplifier side cancel out each other, thereby allowing the received signal amplified by the preamplifier to be accurately distinguished. In addition, the equivalent capacitance device connected to the input terminal of the reference amplifier is formed on the semiconductor chip where the preamplifier, the comparator, and the reference amplifier are formed, thereby making it possible to reduce the size of the module.
More specifically, according to another typical aspect of the present invention, multi-layer wiring technology is used whereby a power supply line is formed with a first wiring layer along the edge of the semiconductor chip, that is, in a region outside of where a signal input pad is formed on the semiconductor chip on which a receiving circuit including the preamplifier, the comparator, and the reference amplifier is formed; whereby a part of the power supply line is provided with a section extended to the input terminal side of the reference amplifier; and whereby the equivalent capacitance device is formed by placing a dummy pad, which is connected to the input terminal of the reference amplifier and comprised in a second wiring layer located above the first wiring layer, in such a manner as to face opposite to the redundant section with an insulating film intermediate between the dummy pad and the redundant section. This makes it possible to make the receiving module more compact in configuration than when an external capacitor is used as the equivalent capacitance device connected to the input terminal of the reference amplifier. Furthermore, the size of the chip is not greatly increased because limited chip space is used effectively. The means described above are extremely effective in reducing the size of a module especially when they are applied to a module that receives multi-channel optical signals.
According to yet another typical aspect of the present invention, the following structure is used. Specifically, an SOI substrate (Silicon On Insulator substrate) is first used as a semiconductor chip on which a receiving circuit including the preamplifier, the comparator, and the reference amplifier is formed. The SOI substrate is formed by laminating semiconductor layers, in which the receiving circuit is formed, on a substrate serving as the base with an insulating film intermediate between the substrate and the semiconductor layers. Then a first supply voltage terminal is connected to the substrate serving as the base, and a second supply voltage terminal is connected to the semiconductor region where devices comprising the receiving circuit including the preamplifier, the reference amplifier, and the like are formed. The capacitance formed between the semiconductor region and the substrate serving as the base is configured to function as a bypass capacitor for stabilizing the supply voltage.
In a receiving circuit constituting the receiving module, the preamplifier and the reference amplifier are arranged in accordance with the pitch of each diode in the photodiode array. Therefore, amplifiers for each channel will be formed in a region with relatively large room. Thus, the capacitance formed between the base substrate and the semiconductor region in the SOI substrate where devices comprising the reference amplifier are formed has a relatively large capacitance value large enough for the capacitance to function as a bypass capacitor. Therefore, it is possible to realize a bypass capacitor having a large capacitance value without increasing the original chip size.
In addition, preferably, the preamplifier and the reference amplifier are formed by a current-input and voltage-output type amplifier circuit comprising: a grounded-emitter type bipolar transistor with its base terminal connected to a signal input terminal; a collector resistance connected between the collector terminal of the transistor and a first supply voltage terminal; an emitter-follower transistor with its base connected to the collector terminal of the transistor; an emitter resistance connected between the emitter terminal of the emitter-follower transistor and a second supply voltage terminal; and a feedback resistance connected between the emitter terminal of the emitter-follower transistor and the signal input terminal.
This makes it possible for the preamplifier to operate in a stable manner even when the supply voltage of the circuit is at a relatively low level and to perform amplification for current input with a wide range of current amplitudes. Therefore, the comparator operates accurately even if there are variations between devices in the photodiode array and the like. Also, accurate operation is ensured even if the amplitude of input signals is reduced due to performance changes with time. As a result, the reliability of the module is improved.
Moreover, it is desirable that a microlens array with a plurality of lenses that are capable of collecting the light of a received optical signal for each photodiode and irradiating the photodiode with the light be provided in the proximity of the photodiode array. Since the receiving circuit, the photodiode array, and the microlens array are formed integrally with one another, this makes it possible to further reduce the size of the optical interconnection receiving module, which processes optical signals on a plurality of channels.
Furthermore, a fiber fixing section for fixing one end of an optical fiber is provided for the microlens array in one section of the package where the photodiode array and the receiving circuit are mounted. This makes it possible to connect and fix an optical fiber to the module very easily.